Semiconductor fabrication routinely relies on a number of fabrication processes to manufacture integrated circuit devices, or chips. Photolithography, for example, is routinely used in the creation of patterned layers of various materials on the surface of a substrate such as a semiconductor wafer. For example, in order to create a layer of conductive wires on the surface of a substrate, a thin film of metal may be deposited across the substrate followed by the deposition of a temporary layer of a photosensitive material referred to as photoresist. Photolithography is then used to form a desired geometric pattern in the photoresist by exposing the photoresist to light projected through an optical mask having apertures matching the desired geometric pattern. Exposure of the photoresist to light modifies the chemical properties of the exposed portions such that when the photoresist is developed, selected portions of the photoresist material are removed and an area of the surface of the substrate corresponding to the desired geometric pattern is accessible through the photoresist. Etching may then be performed to remove the portions of the thin film not covered by the photoresist, followed by removal of the remaining photoresist material, leaving a thin film of metal patterned according to the desired geometric pattern.
Optical masks used in photolithography are typically generated using computer software, and are often manufactured themselves using a photolithographic process that etches apertures into an opaque film disposed on a transparent substrate. The geometric patterns for the masks are typically stored in electronic form as mask files, which are fed to a stepper or scanner to pattern the mask with the desired geometric pattern.
In addition, an alternative photolithography technique referred to as maskless lithography eliminates the use of an optical mask, and instead scans a narrow beam over the photoresist to directly write the desired geometric pattern into the photoresist. As with the mask files used to create optical masks, the electronic data that controls the maskless lithography process to write a desired geometric pattern into the photoresist is also referred to as a mask file, and is generated using computer software.
A common feature of traditional mask-based and maskless photolithography is that they are typically binary processes. Put another way, with both mask-based and maskless photolithography, the photoresist layer is effectively opaque for any areas of a substrate that are covered by the photoresist material, while for areas where the photoresist material has been removed, the underlying areas of the substrate are fully exposed for subsequent processes such as etching.
Grayscale lithography, in contrast, is a technique used to expose different areas of a photosensitive substrate to various intensities of light. By controlling the amount of light each region of the substrate is exposed to, the depth at which photoresist is developed can be carefully controlled. This process can result in the creation of complex three dimensional (3D) structures in photoresist simply from using a single exposure lithographic technique, which may then be copied to an underlying substrate using an etching technique such as deep reactive-ion etching (DRIE) to generate a topography including complex three dimensional shapes on the surface of a substrate. Grayscale lithography is of particular interest for use in fabricating microelectromechanical systems (MEMS) such as micro- or nano-scale sensors and actuators, microfluidics, and microoptical components like microlenses, diffractive elements or hybrids.
While grayscale lithography provides an extremely versatile tool for MEMS fabrication, grayscale lithography is a relatively underutilized process. In part, the underutilization of grayscale lithography is due to time and effort required to create mask files suitable for defining complex 3D topographies, e.g., to control a maskless laser lithography system such as a laser pattern generator. Many such systems utilize mask files that define multiple layers, with each layer corresponding to a different level of exposure, and with geometric objects defined in each layer to define the desired three dimensional profile. Particularly for complex topographies, the manpower required to generate the mask files can be cost and time prohibitive. In addition, in many instances, the calculations required to generate the mask files are too complex to be completed by hand.
Therefore, a significant need exists in the art for an improved method for creating mask files used in connection with grayscale lithography.